MeasureIt

Measurelt is a small extension that lets you measure any region of your browser with pinpoint accuracy.

To do this just click the small icon at the bottom left corner of the browser (once you have it installed), then use the mouse to choose the area to be measured. With some keyboard shortcuts, like using the left and right arrows to adjust the box by a single pixel, you can make your selection even more precise.

Currently the extension is in its very early stages of development and doesn't let you use certain basic features, like changing the unit of measurement (which is always in pixels). However, the MeasureIt website says that the tool is under active development, meaning everything should be on its way.

Measurelt should prove a interesting tool for graphic designers as it saves a lot of time when measuring a website's elements.